We did not create the scenario according to the number of models in the box, but it is true we are used to the Nightmares+Fiend routine. There is no rationale to the 11th Nightmare. We started playing that way and it worked well.
One of our testers likes to place 1 nightmare in the closest room to the elevator. It usually get killed on the very first turn, but can be annoying if the commandos roll badly. I play more conservatively.
I scored a win on this scenario today with the commandos., but my partner had 1 50% roll to win the game, and rolled poorly.
We are still testing various initial doors setups.
I am also thinking about reducing the time limit to 8-10 turns.